As the need for faster and more powerful integrated circuits (ICs) grows, feature sizes on semiconductor waters for the ICs continue to decrease. Successful production of the ICs may require that features on a photomask used in a lithography system to manufacture the ICs have desired and uniform sizes. Photomask manufacturers routinely evaluate feature sizing performance on photomasks by measuring specific features in order to ensure that the photomasks include features that have the desired and uniform sizes. The features that are evaluated may be generally referred to as critical dimensions (CDs) and may be obtained through the use of metrology tools, such as optical systems or scanning electron microscopes.
Today, most measurements are obtained by using optical systems. These optical systems are typically calibrated by creating an initial magnification factor, also known as a pitch factor, that may be modified through another calibration filter that more accurately aligns the initial magnification factor with a recognized feature width reference. However, as feature sizes have decreased below resolution limits of currently available optical CD measurement tools, the use of scanning electron microscope (SEMs) has increased.
Typically, SEMs are calibrated through an appropriate magnification factor that may be determined through measurement of feature pitch on a reference target, such as a photomask or a semiconductor wafer. The reference target may contain features that have known sizes and represent the CDs for a specific manufacturing process. However, an initial calibration for a SEM based on pitch may not be adequate since CD measurements depend on the accuracy of how the SEM measures the widths of features on a photomask or semiconductor wafer.
The measurements provided by a SEM, however, may be inaccurate for tool calibration. When a surface of a reference target is imaged using a scanning electron beam in a SEM tool, any transient hydrocarbons in the chamber are attracted to the interface of the surface and the beam. As a result, a thin layer of carbon material is deposited on the imaged area of the surface. The carbon deposited during one measurement may be negligible but if a location is measured numerous times (e.g., a location on a reference target used to evaluate the initial calibration of the SEM), a thicker layer of carbon may build up on the surface of the target. The carbon layer may change the size of the feature, and, therefore, the surface characteristics at the measured location. Unless the change in feature size over time is taken into consideration, any measurement of the feature size will be different than the size determined during the initial calibration process and will affect the accuracy when the SEM is re-calibrated.
Furthermore, re-calibration of optical systems and SEMs may be time consuming. Typically, the re-calibration process may involve evaluating the measurement of each relevant feature on the reference target. For example, a reference target may include between eight to twelve reference features over a desired calibration range. The re-calibration process may take numerous hours of the tool's time if each of the features is measured and, therefore, decrease the number of production photomasks or wafers that may be measured during a given time period.